Vehicles commonly have headlamps that provide low and high beam lighting for driving at night. High beam lighting provides a substantial amount of light aimed so as to enable the driver to see greater distances on dark roads, while low beam lighting provides a lesser amount of light aimed closer to the vehicle for driving on lighted streets or to prevent dazzling drivers of oncoming vehicles. Separate incandescent and/or halogen lamps may be utilized for low and high beam headlamps. Alternatively, a single lamp may have a plurality of selectable filaments for high and low beam operation, or a high beam lamp may be dimmed for low beam operation.
Vehicle designers are increasingly turning to high intensity discharge (“HID”) lamps for use in headlamp systems due to their high efficiency in comparison to incandescent and halogen lamps. A typical characteristic of HID lamps is that they must be operated at a generally fixed power level for proper operation. This inflexibility makes dimming of an HID high beam headlamp for low beam operation impractical. To avoid the complexity and expense of utilizing two HID systems to cover both high beam and low beam operation, vehicle designers have incorporated into headlamp assemblies various types of movable shades proximate a single HID lamp. The shades are typically moved to predetermined positions proximate the lamp for low beam operation, partially shielding and/or redirecting light emitted from the lamp. Accordingly, only a portion of the total amount of light emitted by the lamp reaches a reflector to be directed out of the headlamp. When high beam operation is desired, the shade is moved away from the HID lamp, allowing a greater portion of the light emitted by the lamp to reach the reflector and be directed out of the headlamp.
As a safety enhancement many vehicles now include daytime running lights (“DRL”) in addition to low and high beam headlamps. As the term implies, DRL are normally operated in substantially daylight conditions. A vehicle equipped with DRL is more likely to be noticed by other drivers, thereby reducing the probability of a collision with the vehicle.
Daytime running lights that use incandescent or halogen lamps as a light source are typically formed from the vehicle's low or high beam headlamps. For DRL operation with high beam headlamps, the headlamps are operated at a reduced intensity. Alternatively, low beam headlamps may be operated at either full or reduced power for DRL operation. Such systems are designed to automatically function as DRL when the vehicle is started and to be overridden when regular high or low beam headlamps are activated.
DRL presents a challenge for HID-based vehicle lighting systems, since HID lamps must be operated at a relatively fixed power level and are not easily dimmed, as discussed above. This limitation often drives the use of a separate DRL unit, adding cost and complexity to the vehicle. Thus, there is a need for a way to avoid the expense and complexity of a separate DRL system for vehicles having headlamps that use HID lamps as a light source.